The present invention relates to apparatus, and also to a method, for optically inspecting articles in order to obtain maximum contrast. The invention is particularly applicable for use in optical microscopes, in order to enhance the contrast between different materials or layers in the inspected articles, and the invention is therefore described below with respect to such an application.
The optical microscope is a well known and widely used instrument for observing small objects and minute details of surfaces in a wide variety of fields. It is particularly useful for visual inspection of electronic products, such as integrated circuits, multi-chip modules, flat panel displays, and the like, all characterized by finely detailed surfaces.
Several well known contrast enhancing techniques in microscopy use polarizers in the optical path. Accordingly, most commercially available microscopes may be equipped with a polarizer in the illuminating system, and an analyzer in the imaging system, with the polarizer rotatable around the illumination axis so that the orientation of the polarization axis is continuously variable within an angular range of about 90.degree.. Such microscope illuminating systems may also include a phase compensator, or a phase retardation plate, which introduces a phase shift of 360.degree. between the s- and p- polarization components.
However, some important types of articles or workpieces are hard to inspect with the prior art microscopes because the known contrast enhancement techniques fail to yield adequate contrast. One type of such article is a glass substrate coated with an indium-tin oxide (ITO) patterned layer about 100 nm thick, overcoated with a polyimide (PI) film about 80 nm thick, which is used in liquid crystal displays (LCD). These articles should be inspected particularly for regions of possible absence of the polyimide film. However, no microscopic technique is known to the inventors which adequately differentiates between regions coated with such a thin layer, and uncoated regions.
Another example of hard-to-inspect articles is silicon coated with a silicon dioxide layer of less than 50 .mu.m thickness, a common stage in the fabrication of microelectronic devices. However, the known microscopic techniques are unable to provide adequate contrast for examining the silicon dioxide layer.
There is therefore a critical need for optical inspection apparatus which and method which enhance the contrast between different materials in the article even when extremely thin layers or films are involved.